Ammonia-chain clusters:: Vibronic spectra of 7-hydroxyquinoline•(NH3)2

Mass- and isomer-selected S-1<--S-0 resonant two-photon ionization and S-1-->S-0 fluorescence spectra were measured for the 7-hydroxyquinoline.(NH3)(2) [7HQ.(NH3)(2)] and d(2)-7-hydroxyquinoline.(ND3)(2) clusters cooled in supersonic expansions. UV/UV hole burning measurements prove that a single cluster isomer is formed. Ab initio self-consistent field and density functional calculations predict that the most stable cluster form has an "ammonia wire" hydrogen bonded to the -OH and N groups of the cis-7HQ rotamer. The experimental S-0 and S-1 frequencies are in very good agreement with the calculated normal mode frequencies for both the normal and deuterated ammonia-wire clusters. S-1<--S-0 excitation leads to contractions of the -O-H.N and NH3.NH3 hydrogen bonds, as well as smaller displacements for the NH3.N(quinoline) stretch and the in plane rotation (or bend) of the ammonia dimer relative to 7HQ. The coupling of these modes to the S-1<--S-0 electronic excitation indicates that hydrogen bond contractions in the excited state are important and may be prerequisite for the S-1 state proton transfer processes that occur in the larger 7HQ.(NH3)(n) (ngreater than or equal to4) clusters. The calculated electron density differences upon S-1<--S-0 excitation show large pi-electron flows on the 7HQ moiety. However, the sigma-electronic rearrangements that directly drive the hydrogen bond rearrangements are one to two orders of magnitude smaller. (C) 2003 American Institute of Physics.